AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1997, No. 2
Soil testing is the cornerstone of an effective fertility program. Unfortunately, many acres of winter wheat are planted and fertilized without a soil test. Planning fertilization without soil test data is largely guesswork. Soil test information is only as good as the sample collected. If samples are not representative of the area in question, the value of the resulting data and recommendations is compromised. A great deal of attention has recently been focused on spatial variability in soil properties. Intensive (grid) sampling to address and delineate this variability has been adopted in some cases. Where more conventional composite sampling is used, care should be taken to assure that enough samples are collected to provide a sample that is representative of the entire area or field. Also, a reasonable yield goal should be established.
Nitrogen performs many vital functions in the wheat plant and is directly related to protein and chlorophyll content. Wheat requires 2 to 2.5 pounds of nitrogen per bushel of grain; if grazed, one pound for each 3 pound of animal gain. Inadequate nitrogen in the fall may result in an underdeveloped root system. As the season progresses, shortages of nitrogen may result in less tillering, reduction in head size, poor grain fill, and low protein content. Adequate nitrogen must be available to the wheat plant at all stages of development. Splitting nitrogen applications is recommended to improve use efficiency, minimize risk to investment, and to safeguard the environment. Topdress applications should be made prior to jointing to maximize production efficiency. Timing, placement, and nitrogen source should be managed to fit climatic conditions, soil type, and tillage system.
Adequate phosphorus fertility is associated with increased tillering and grain head numbers, reduced winter kill, maximum water use efficiency, hastened maturity, and lower grain moisture at harvest. Winter wheat requires about 0.6 to 0.7 pound P2O5 per bushel of grain. Because phosphorus is relatively immobile in soils, banded or starter applications are often most effective in soils testing low to medium. Even in high testing soils starter applications help plants get established more quickly. Banded phosphorus also helps young plants overcome the adverse effects of soil acidity. Broadcast phosphorus should be incorporated to improve positional availability. Soil test phosphorus should be increased to high levels and maintained for optimum long term fertility.
Potassium in wheat production is associated with increased moisture and nitrogen use efficiency and decreased incidence of disease and lodging. The requirement for potassium is approximately equal that of nitrogen. Placement of potassium is not as critical as phosphorus since it is more mobile in soils. Depending on the potassium source, placement with the seed should be limited since stand reduction may occur as a result of salt injury. Split applications should be made on deep sandy soils to increase use efficiency.
Secondary and micronutrients should not be neglected, and applications should be based on soil tests and plant analysis.
Plant nutrients are among the few controllable inputs affecting wheat production. Therefore, every effort should be made to optimize their use. This requires early planning, soil testing and consideration of fertilizer application timing, placement and rate.